1. Field of the Invention
The present invention pertains to a zoom lens system, and more particularly to a zoom lens system having a high aperture ratio and capable of high magnification and appropriate for a still camera, video cameras etc.
2. Description of the Prior Art
Conventionally, in a zoom lens system used in a still camera or video cameras it has been desired to increase the aperture ratio and magnification while reducing the number of lens elements in the entire system in order to make the camera less costly and more compact. In response to this need, a technology has been proposed that reduces the number of lens elements in the entire zoom lens system by employing a gradient index lens (GRIN lens) while performing aberration correction.
For example, Japanese Laid-Open Patent Application 6-337347 proposes two types of zoom lens systems comprising, from the object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and fourth lens unit having a positive refractive power. In one of the examples, the second and fourth lens units comprise a spherical GRIN lens and a spherical homogeneous lens, and in the other example, the second lens unit comprises a spherical GRIN lens and two homogeneous lenses having an aspherical surface, and the fourth lens unit comprises a GRIN lens and a spherical homogeneous lens.
Japanese Laid-Open Patent Application 4-369611 proposes a zoom lens system comprising, from the object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a positive refractive power, where a spherical GRIN lens is used in one of said lens units such that either the first, third or fourth lens unit consists of a single lens element.
However, the zoom lens system discussed in said Japanese Laid-Open Patent Application 6-337347 has the problem that, because the GRIN lens is a flat lens, there is an insufficient degree of freedom to perform aberration correction, and adequate performance cannot be maintained. As a result, the problem arises that the number of lens elements in the lens units not containing a GRIN lens, such as the first lens unit, increases accordingly.
The zoom lens system discussed in Japanese Laid-Open Patent Application 4-369611 has the problem that the second lens unit comprises a large number of lens elements. In said zoom lens system, examples in which the first, third or fourth lens unit consists of a single lens element using a GRIN lens are proposed, but the second lens unit, which has the largest aberration correction effect to reduce the size of the zoom lens system, does not comprise two or fewer lens elements.